(a) Field of the Invention
The present invention relates to a displaying device. More specifically, the present invention relates to an organic electro-luminescence (herein also referred to as EL) light-emitting display using the electro-luminescence of an organic material.
(b) Description of the Related Art
The organic EL light-emitting display, which is a device for electrically exciting an organic fluorescent material to emit a light, drives n×m organic light-emitting elements arranged in a matrix to represent an image.
The organic light-emitting element, also called “organic light-emitting diode (herein also referred to as OLED)” because of its diode characteristic, has a structure of an Indium Tin Oxide (herein also referred to as ITO) anode, an organic thin film, and a cathode electrode (e.g., metal) layer. The organic thin film has a multi-layer structure that includes a light-emitting layer (herein also referred to as EML), an electron transport layer (herein also referred to as ETL), and a hole transport layer (herein also referred to as HTL) to keep electrons and holes in good balance and enhance the light-emitting efficiency. The organic thin film may also include an electron injecting layer (herein also referred to as EIL) and a hole injecting layer (herein also referred to as HIL). The n×m organic light-emitting elements are arranged in a matrix to form an organic EL display panel.
There are two driving methods for the organic light-emitting elements: the passive matrix method and the active matrix method using thin film transistors (TFTs.) The passive matrix method forms anode and cathode lines arranged orthogonal to (or to cross over) each other and select the anode and cathode lines to drive the organic light-emitting elements. The active matrix method sequentially turns on a plurality of TFTs coupled to data lines and scan lines to drive the organic light-emitting elements.
Hereinafter, a pixel circuit of a general active matrix organic EL display device will be described.
FIG. 1 is an equivalent circuit diagram of one of n×m pixels, i.e., a pixel on a first row and a first column of the n×m pixels.
Referring to FIG. 1, a pixel 10 includes three sub-pixels 10r, 10g and 10b, which have organic light-emitting elements OLEDr, OLEDg and OLEDb to emit red (R), green (G) and blue (B) lights, respectively. In a stripe arrangement structure, the sub-pixels 10r, 10g and 10b are coupled to independent data lines D1r, D1g and D1b, and a common scan line S1.
The red sub-pixel 10r includes two transistors M11r and M12r and a capacitor C1r for driving the organic light-emitting element OLEDr. Likewise, the green sub-pixel 10g includes two transistors M11g and M12g and a capacitor C1g for driving the organic light-emitting element OLEDg, and the blue sub-pixel 10b includes two transistors M11b and M12b and a capacitor C1b for driving the organic light-emitting element OLEDb. The connections and operations of the sub-pixels 10r, 10g and 10b are substantially the same. As such, the connection and operation of only the sub-pixel 10r will be described below by way of an example.
The driving transistor M11r is coupled between a power supply voltage VDD and the anode of the organic light-emitting element OLEDr to transfer a current for light emission to the organic light-emitting element OLEDr. A cathode of the organic light-emitting element OLEDr is coupled to a voltage VSS, which is lower than the power supply voltage VDD. The amount of current of the driving transistor M11r is controlled by a data voltage applied through a switching transistor M12r. Here, a capacitor C1r is coupled between a source and a gate of the transistor M11r to sustain (or maintain) the applied voltage for a predetermined time period. The transistor M12r has a gate coupled to a scan line S1 for transferring an on/off selection signal (or select signal), and a source coupled to a data line D1r for transferring a data voltage corresponding to the red sub-pixel 10r. 
In operation, when the switching transistor M12r is turned on in response to the selection signal applied to its gate, a data voltage VDATA from the data line D1r is applied to the gate of the transistor M11r. Then, a current IOLED flows to (and/or through) the transistor M11r according to a voltage VGS charged between the gate and the source by the capacitor C1r. The organic light-emitting element OLEDr then emits a light according to the current IOLED Here, the current IOLED flowing to the organic light-emitting element OLEDr is given by the following Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            where VTH is the threshold voltage of the transistor M11r; and β is a constant.
As can be seen from the Equation 1, in the pixel circuit of FIG. 1, a current corresponding to the data voltage is supplied to the organic light-emitting element OLEDr, causing the organic light-emitting element OLEDr to emit a light with a brightness corresponding to it. The data voltage applied has a multilevel value in a defined range so as to represent a predetermined brightness gradation.
In the organic EL display device, one pixel 10 includes three sub-pixels 10r, 10g and 10b, each of which has a driving transistor (M11r, M11g, or M11b) for driving an organic light-emitting element, a switching transistor (M12r, M12g, or M12b), and a capacitor (C1r, C1g, or C1b). Each sub-pixel also has a data line (D1r, D1g, or D1b) for transferring a data signal, and a power line for transferring the power supply voltage VDD.
Therefore, such a construction requires a large number of elements such as transistors and capacitors formed in one pixel and hence a large number of interconnections to transfer voltages or signals, so there is a considerable difficulty in arranging all the component elements in one pixel area.